Implantable medical devices (IMDs) have many functions including the delivery of therapies to cardiac patients, neuro-stimulators, muscular stimulators, and others. For purposes of this application reference will be made only to implantable cardiac devices, it being understood that the principles herein may have applicability to other implantable medical devices as well.
An implantable cardiac device (ICD) may be a device commonly referred to as a pacemaker, which is used to stimulate the heart into a contraction if the sinus node of the heart is not properly timing, or pacing, the contractions of the heart. Modern cardiac devices also perform many other functions beyond that of pacing. For example, some cardiac devices may also perform therapies such as defibrillation and cardioversion as well as providing several different pacing therapies, depending upon the needs of the user and the physiologic condition of the user's heart. For convenience, all types of implantable cardiac devices will be referred to herein as ICDs, it being understood that the term, unless otherwise indicated, is inclusive of an implantable device capable of administering any of a number of therapies to the heart of the user.
In typical use, an ICD is implanted in a convenient location usually under the skin of the user and in the vicinity of the one or more major arteries or veins. One or more electrical leads connected to the pacemaker are inserted into or on the heart of the user, usually through a convenient vein or artery. The ends of the leads are placed in contact with the walls or surface of one or more chambers of the heart, depending upon the particular therapies deemed appropriate for the user.
One or more of the leads is adapted to carry a current from the pacemaker to the heart tissue to stimulate the heart in one of several ways, again depending upon the particular therapy being delivered. The leads are simultaneously used for sensing the physiologic signals provided by the heart to determine when to deliver a therapeutic pulse to the heart, and the nature of the pulse, e.g., a pacing pulse or a defibrillation shock.
There has been recent interest in development of implantable defibrillators that may be inserted entirely subcutaneously or sub-muscularly, having no leads or electrodes within the thoracic cavity. The elimination of transvenous or epicardial leads is believed likely to allow for implant of the devices by a wider range of physicians, in some cases at a lower cost than traditional ICDs. Absence of transvene or epicardial leads may reduce acute and long term complications. Such devices, are therefore believed to offer the opportunity for increased levels of use, particularly for prophylactic implant. US Application Publication Nos. 2002/0042634, 200200068958 and 2002/0035377 to Bardy et al., are exemplary of current thinking with regard to such subcutaneous ICDs. Additional subcutaneous ICDs are disclosed in US Application Publication No. 20020082658 by Heinrich et al. and PCT publication WO/04043919A2 by Olson. All of the above cited applications and publications are incorporated herein by reference in their entireties.
One potential problem associated with the sensing of the physiologic signal from the heart in both the transvenous systems and the subcutaneous systems relates to what is often referred to as “false positive” and “false negative” detections. The most widely accepted detection algorithm is based on the rate of depolarizations of the ventricles, or simply on “heart rate”. Such algorithms rely on detecting events based upon signals obtained between two electrodes positioned within or on the heart. If the number of detected events per a given time is greater than a preset value, then the device charges an energy storage capacitor and then shocks the heart; otherwise no shock is delivered.